The biodefense agent Staphylococcus aureus enterotoxin is regarded as a superantigen (SAg) because it is not processed as conventional peptide and is an incredibly powerful T cell stimulant. Enterotoxins can cause lethal toxic shock in humans and intentional aerosol exposure of S. aureus enterotoxin has the potential to incapacitate large groups of people. The consequences of such a biological attack can lead to severe respiratory illness in exposed individuals. Notwithstanding, certain pulmonary diseases in human patients have recently been associated with the presence of SAg, and stimulation of T cells with SAg in the respiratory tract drives an inflammatory cascade resulting in severe pathological outcomes in the lung. This includes pulmonary inflammation, vascular permeability and alveolitis. Although this pathogenic response involves SAg-specific T cells, the underlying mechanism of disease initiation and mediators of lung inflammation are unclear. Our data show that innate cell recruitment and their activation in lung are dependent upon TCR Vp bearing T cells and occurs rapidly after exposure. A new finding clearly demonstrates this point since only a few hours after SAg inhalation y.ST cells begin to synthesize I L-17a. Unexpectedly, 1L-I7a production was shown to be dependent on the presence of TCR Vp T cells. The role of SAg-specific IT cell sub-populations involved in controlling Y 5 T cell activation will be investigated in Aim 1. A consequence of early innate cell activation is pulmonary tissue injury, and by proteomic mining of bronchoalveolar lavage fluid we detected intracellular proteins that may have come from damaged lung cells. In Aim 2 we will purify and characterize a protease-sensitive factor that we detected in bronchoalveolar lavage fluid which has characteristics of a DAMP. Our goal is to identify this molecule and validate its ability to stimulate cytokine release by defining its cellular target and signaling properties. Lastly, an outcome of DAMP activity is intensification of inflammation, and in this regard our new data show a potential role for the IL-33 pathway. Aim 3 will test how S. aureus enterotoxin inhalation impacts the IL-33 pathway and if it can be modulated to develop countermeasures against a pulmonary crisis. RELEVANCE (See instructions): The study of enterotoxins released from Staphylococcus aureus is an important area of research due to the role these proteins play in inducing very strong immune responses. The nature ofthese proteins allows T cells to respondlvigorously which can lead to serious illness in people for which there is no clear treatment. This proposal will address important issues centered on understanding the initiation of these responses.